1. Field of the Invention
The present invention relates to a semiconductor hetero-structure having a low dislocation density, and a method for producing the same.
2. Description of the Prior Art
An attempt has recently been made to grow a GalnAs/AlInAs type HEMT on a GaAs substrate, which is cheaper than an InP substrate and is also superior in strength, in place of an InP substrate.
However, the lattice constant of GaAs is 5.6533 .ANG. while each lattice constant of Ga.sub.0.47 In.sub.0.53 As and Al.sub.0.48 In.sub.0.52 As to be grown on the InD, is 5.8688 .ANG.. Therefore, there is always a lattice mismatch of about 4% between GaAs the Ga.sub.0.47 In.sub.0.53 As or Al.sub.0.48 In.sub.0.52 As and thus Ga.sub.0.47 In.sub.0.53 As grown directly on a GaAs substrate has a large concentration of crystal defects caused by the lattice mismatch.
In order to solve such a problem, for example, as shown in a cross-sectional view of FIG. 16, there are grown, in order, on a GaAs substrate 1, an undoped GaAs layer 13 having a thickness of 20 nm to an undoped AlAs layer 14 having a thickness of 20 nm, and an undoped AlInAs layer 15 having a thickness of not less than 330 nm, wherein the composition ratio x of Al.sub.1- .sub.x In.sub.x As is changed in steps within the range from 0.15 to 0.45, to alleviate the lattice mismatch. Thereafter an undoped Al.sub.0.55 In.sub.0.45 As barrier layer 11 having a thickness of 20 nm, an undoped Ga.sub.0.55 In.sub.0.45 As channel layer 3 having a thickness of 30 nm, an Al.sub.0.55 In.sub.0.45 As first spacer layer 4 having a thickness of 2 nm, a Si-doped Al.sub.0.55 In.sub.0.45 As electron supplying layer 8 having a thickness of 2 nm, and an Al.sub.0.55 In.sub.0.45 As Schottky spacer layer 8 having a thickness of 10 nm are sequentially grown. Finally, there is formed an ohmic contact layer 16 of a three layer structure having GalnAs/AlInAs/GaInAs, thereby forming GalnAs/AlInAs HEMTs on the GaAs substrate while inhibiting generation of crystal defects caused by lattice mismatch (Journal of Crystal Growth 150 (1995) pp1230-1235).
Such a method of changing the composition ratio x of Al.sub.1-x In.sub.x As gives HEMTs formed on the GaAs substrate decreased lattice mismatch and have improved electron mobility of 8,800 cm.sup.2 /Vs at room temperature, substantially the same as that an HEMT formed on an InP substrate. However, the HEMT formed on the GaAs substrate still has a worse surface condition than an HEMT formed on an InP substrate, causing diffuse reflection and a pattern of crosshatching caused by microcracks and crystal defects observed in a surface differential interference microphotograph. Such crystal defects make the HEMT quality worse, according to a test of applying a current between the source and drain at 190.degree. C., with respect to transconductance (gm) and saturated drain current (Idss), the HEMT formed on the InP substrate showed no deterioration during 500 hours or more while the HEMT formed on the GaAs substrate showed deterioration such as a decrease to 90% or less during 3 hours, which means it is difficult to put the HEMT formed on the GaAs substrate into practical.
Accordingly, a first object of the present invention is to provide an AlInAs/GaAs or GaInAs/GaAs hetero-structure suitable for an HEMT.
Further, a second object of the present invention is to provide a method of preparing an AlInAs/GaAs or GaInAs/GaAs hetero-structure suitable for an HEMT.